On a serious note… what is the exact amount of time a duty cycle is based off? I always see examples like 10 minutes every 30 or 20 every hour… but what is the test definition? I guess its off topic but it would be nice to have a clear answer somewhere.

Duty cycle is the percentage of time the compressor will run at a specified pressure, or less than the specified pressure, during a 1 hour period. Running the compressor at a higher pressure shortens the duty cycle.

50% duty cycle means it will run 30 minutes out of an hour whether that is continuous or not.

Appreciate it. Just wanted an expert to say the right interval. I always find different numbers everywhere since people apply the percentage to all sorts of time periods.
Good to actually hear that the standard is 1 hour. I always figured… but no word from anyone directly

To further describe, VIAIR tests all compressors at an ambient temperature of 72 degrees, typically at 100 PSI to determine duty cycle. Duty cycle can be expressed in percentage format as:

Compressor on time divided by (on time + off time) = Duty Cycle Percentage.

All of this information has been listed in every VIAIR catalog since the dawn of VIAIR’s existence.

Lance,
Is is accurate to say that for a 33% duty cycle compressor(at 100psi, at 72 deg) can run for 20 minutes straight? If not, is there information anywhere that states what the continuous “on time” is?

Ok, thanks. I just wish I had done a little more research before diving into the whole train horn craze. Of course, I would not have had any issues if 4WheelParts had not messed up my order and sent me a 20 gallon tank instead of the 12 gallon tank I paid for. I think I finally understand why the larger tank makes such a difference. If I understand correctly, the pressure drop from 200 psi to 165 psi represents a particular volume of air. With a 20 gallon tank, the volume of air that has to be released to drop the pressure from 200 psi to 165 psi is almost twice the amount of a 12 gallon tank. The recovery time to get the tank back up to 200 psi on a 20 gallon tank is going to be almost twice as long as the recovery time on a 12 gallon tank.

On that note, I think that Viair (and Viair customers) could really benefit from a chart on it’s website that shows all of the compressors and how they compare and what the benefits of one is over the other for specific applications. Like, which ones would be best for running air tools or air suspension or horns or airing up tires. I don’t know if the dual 480C setup was the best choice for me. It would just be nice to make an educated decision about these things. I read all the charts on Viair’s website, but to be honest, it’s hard to make heads or tails out of all those charts. One simple to read chart showing all the compressors, their specs, and suggested applications would be great.

Also, I understand that if a compressor has a 100% duty rating at 100 psi, that duty rating is cut in half by running 200 psi. When my 20 gallon tank falls below 165 psi, the compressors come on and take about 4-5 minutes to bring the pressure back up to 200 psi. As little as I use the horn, the compressors usually only come on about twice a day. Should my compressors be ok under these conditions or should I get a lower rated pressure switch, like a 150 psi?

That helps with duty cycle, but it really doesn’t help a person decide which compressor is right for a certain application. You have to remember, the average person installing one of these compressors is not a professional and doesn’t really understand all the technical jargon. I was referring to something more like a chart that shows all the different compressors on the same page with all the specs and suggested applications for each model. You know, something that would actually make sense to a novice like me. Something that could help a person make an educated decision.

I’m not sure it’s that easy. There are so many different applications and compressors. The best solution is to call and ask questions. That way you can tell the manufacturer your application and they can give you some solutions.

It has but, if I’m not mistaken, examples have included times other than an hour… which doesn’t reflect the compressor’s true duty cycle.

I just pulled out a random Viair 4-fold giving 10/40 minutes as a 25% as an example. If the testing environment is actually an hour it should be 15/60 minutes otherwise it seems deceiving. That said I do understand that regarding it as meaning 10/40 minutes is safe since that says don’t run your compressor more than 10 minutes without a half hour wait period when actually in factory tests (presuming both are 72 degrees for the sake of argument) when it can actually take up to 15 minutes no problem; however, making it a relative percentage that can be applied to any time period, like in the example, gives a false sense of accuracy.

An owner could just as easily presume it means 25% of any given amount of time… lets say 3 hours and run their compressor for 45 minutes constant presuming its safe as long as they give it a 2 hr 15 minute wait period. Obviously that’s out of context and exaggerated on purpose. I doubt anyone would really justify that in their head, but in the literature it doesn’t say “1 hour” anywhere.

Edit: I’d also like to add that the term 100% is deceiving in the manor too. Since 1 hour isn’t stated, 100% suggests that a compressor can run endlessly. Now surely anyone could guess that this isn’t the case, but the subject is never touched on. Should an owner translate 100% to mean a compressor can run for 1 hour exactly without hurting its service life beyond the MBTF (mean time before failure) for the unit?

I also know that this is an exaggeration and it would be difficult to run a compressor for over an hour in regular conditions, but if an owner doesn’t recognize this as a problem and should, then it should be stated somewhere.

Verbatim:

Duty cycle refers to amount of time a compressor can be operated in a given time period, at 100 PSI, at a standard ambient temperature of 72 degrees F.

Duty cycle is commonly expressed in percentage format as compressor on time / (compressor on time + compressor off time) %

For Example: A compressor that is rated for 25% duty cycle means that the compressor can be operated for 10 minutes on and 30 minutes off = 10 minutes / (10 minutes + 30 minutes) = 10 minutes / 40 minutes = 25% duty cycle

(End Quote)

The reason I asked the question was that it states nowhere that the testing period for duty cycle was in fact 1 hour (at least not in the literature I’ve read).

Since duty cycle is merely a percentage, it works with any time period. Yes, a 25% duty cycle would be equivalent to 10/40 or 15/60 or 30/120 or 60/240 and so on and so forth. That’s the beauty of a percentage. You can take the amount of on time and divide it by the total time and you will get a percentage. If that percentage is less than the rated duty cycle of the compressor, you’re good. If it’s higher, then you will be decreasing the life of the compressor.

Those points are well taken. I’ve also heard some duty cycles were based only on the first hour. This refers to other brands as I can’t speak for Viair.

08FX4SuperDuty,
You MUST account for the max continuous run time before the compressor needs a break. If I have a 25% duty cycle compressor, there’s no way I can run it for 3 hours straight(at 72 deg at 100 psi) even if I gave it the whole weekend to cool down. duty cycle = run/(run + rest) = 3 hours/INFINITY is less than 25% duty cycle, but the compressor will never make it there.